1. Field of the Invention
The present invention relates to a molten bath-based direct smelting process and an apparatus for producing molten ferrous metal (which term includes ferroalloys), in particular although by no means exclusively iron, from ferrous feed material, such as ores, partly reduced ores and metal-containing waste streams which uses a pre-heated air-based (which term includes oxygen enriched air up to 50% O2) system for post combusting reaction gases (principally being CO and H2) that are generated in the molten bath.
2. Description of Related Art
Without adequate post combustion of reaction gases and subsequent substantial transfer of heat back to the molten bath, such bath based direct smelting processes, particularly those without a pre-reduction stage, become uneconomic and in many cases unworkable because of the endothermic nature of the reduction of ferrous feed materials. While oxygen based post combustion systems (which term includes impure O2 down to 50% O2) as against pre-heated air based systems have advantages such as the minimisation of off gas volumes there is also a significant cost in supplying large amounts of oxygen.
A further requirement is that good post combustion must not be at the expense of re-oxiding significant amounts of molten ferrous metal in the molten bath as the process becomes inefficient, with excessive amounts of solid carbonaceous material being needed to counter such oxidation. As well, excessively high amounts of liquid FeOx (principally FeO) in the molten bath is particularly detrimental to refractory wear in the region between the nominal quiescent surface level of the molten ferrous metal and that of the bath as a whole.
One known molten bath-based direct smelting process for producing molten ferrous metal is the DIOS process. The DIOS process includes a pre-reduction stage and a smelt reduction stage. In the DIOS process ore (−8 mm) is pre-heated (750° C.) and pre-reduced (10 to 30%) in fluidised beds using offgas from a smelt reduction vessel which contains a molten bath of metal and slag, with the slag forming a deep layer on the metal. The fine (−3 mm) and coarse (−8 mm) components of the ore are separated in the pre-reduction stage of the process. Coal and pre-heated and pre-reduced ore (via two feed lines) are fed continuously into the smelt reduction furnace from the top of the furnace. The ore dissolves and forms FeO in the deep layer of slag and the coal decomposes into char and volatile matter in the slag layer. Oxygen is blown through a specially designed lance that improves secondary combustion in the foamed slag. Oxygen jets burn carbon monoxide that is generated with the smelting reduction reactions, thereby generating heat that is transferred to the molten slag. The FeO is reduced at the slag/metal and slag/char interfaces. Stirring gas introduced into the hot metal bath from the bottom of the smelt reduction vessel improves heat transfer efficiency and increases the slag/metal interface for reduction. Slag and metal are tapped periodically.
Another known direct smelting process for producing molten ferrous metal is the AISI process. The AISI process also includes a pre-reduction stage and a smelt reduction stage. In the AISI process pre-heated and partially pre-reduced iron ore pellets, coal or coke breeze and fluxes are top charged into a pressurised smelt reactor which contains a molten bath of metal and slag. The coal devolatilises in the slag layer and the iron ore pellets dissolve in the slag and then are reduced by carbon (char) in the slag. High gas generation rates result in slag foaming. Carbon monoxide and hydrogen generated in the process are post combusted in or just above the slag layer to provide the energy required for the endothermic reduction reactions. Oxygen is top blown through a central, water cooled lance and nitrogen is injected through tuyeres at the bottom of the reactor to ensure sufficient stirring to facilitate heat transfer of the post combustion energy to the bath. The process offgas is de-dusted in a hot cyclone before being fed to a shaft type furnace for pre-heating and pre-reduction of the pellets to FeO or wustite.
Another known direct smelting process for producing molten ferrous metal is the HIsmelt process. One form of the HIsmelt process as described in International application PCT/AU96/00197 (WO 96/31627) in the name of the applicant, comprises:                (a) forming a molten bath of metal and slag in a vessel;        (b) injecting into the bath:                    (i) ferrous feed material, typically metal oxides; and            (ii) a solid carbonaceous material, typically coal, which acts as a reductant of the metal oxides and a source of energy; and                        (c) smelting the ferrous feed material to metal in the metal layer of the molten bath.        
The HIsmelt process also comprises injecting oxygen-containing gas into a space above the bath and post-combusting reaction gases, such as CO and H2, released from the bath and transferring the heat generated to the bath to contribute to the thermal energy required to smelt the metalliferous feed materials.